1. Field of the Invention
The present invention relates generally to method and apparatus for operating internal combustion engines. More particularly, this invention relates to internal combustion engine systems including an engine exhaust gas control system that can raise low engine speed turbocharger boost pressure and can decrease engine warm-up times and can increase the back pressure in the engine exhaust system to provide engine braking.
2. Description of the Prior Art
Fixed geometry turbochargers can be designed to operate efficiently at a particular engine load and speed. However, when operated over a broad range of engine speed and load, the compressor and turbine components are forced to function off their design points; and, consequently, suffer losses in efficiency that adversely affect engine performance. For example, if the turbocharger is matched to an engine at the engine's rated speed, it will run considerably off its maximum efficiency when the engine is "torqued down" to low engine operating speeds. Conversely, if the turbocharger is matched to an engine's low speed range, the turbocharger will have a tendency to "overspeed" when the engine is operated at maximum speed and load.
To prevent overspeeding in turbochargers that have been matched to the low engine speed range, a waste gate is frequently used to bypass exhaust gas around the turbine to limit turbine speed over the high engine speed range. The waste gate, however, allows the escape of exhaust gas energy, which could be better utilized by the turbocharger turbine and results in a substantial loss in system efficiency.
A more efficient system generally known in the trade is one comprising variable geometry components in the turbocharger compressor, the turbocharger turbine, or both. The most common types are variable nozzle vanes ahead of the turbine wheel and/or variable diffuser vanes in the compressor component.
Variable nozzle vanes ahead of the turbine wheel are connected together so that the throat area of each nozzle passage can be reduced over the low engine speed range and increased as the engine speed approaches its maximum, so that the turbocharger speed is kept within a safe operating range. The positioning of the vanes must be precisely controlled by engine speed and load, and they must be freely movable in the hot exhaust gas environment with minimal leakage through clearance spaces.
The various movable devices that have been employed in the turbocharger turbine have been complicated, expensive, and subject to questionable durability. Consequently, they have met with limited commercial success.
A more practical approach is the variable device in the engine exhaust system disclosed in U.S. Pat. No. 3,557,549 to Webster. The system disclosed in this patent employs a flapper valve so positioned in a divided manifold system that it resides in a neutral position at high engine speed and load, but can be moved to a second position where it diverts all engine exhaust gas flow into one passage of a divided turbine casing at low engine speeds. This essentially doubles the flow of exhaust gas through the single turbine casing passage and maintains the turbocharger speed at higher levels than otherwise could be reached at low engine speeds. This device is much simpler than the complicated variable nozzle vane systems and does not require a precise control system for positioning.
The use of a flapper valve to divert exhaust gas allows the turbocharger to be matched efficiently to higher engine speeds when the flapper is in a neutral position. When the engine is operated at low engine speeds, the concentration of full exhaust flow in the single turbine casing passage ahead of the turbine increases the turbocharger rotor speed to provide higher boost pressure to the engine cylinders, allowing the engine to produce more power and torque than otherwise could be obtained.